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9Second International Symposium «Safety and Economy of Hydrogen Transport»
IFSSEHT-2003
KINETICS OF STRUCTURAL TRANSFORMATIONS IN TITANIUM UNDER IRRADIATION BY DEUTERIUM IONS AND POST-IMPLANTATION ANNEALING
A. N. Morozov, I. M. Neklyudov, V. N. Voyevodin, V. G. Koulish1
National Scientific Centre Kharkov Physical-Technical Institute, 1 Akademicheskaya ul., Kharkov, 61108, Ukraine, e-mail: [email protected] 1 G. S. Skovoroda Kharkov State Pedagogical University
The investigations of structural transformations in titanium irradiated by D + ions have been carried out using methods of electronic diffraction and thermodes-orption spectroscopy. The general trends of deuterium accumulation in titanium have been studied under irradiation by deuterium ions depending upon the irradiation dose and temperature; the limiting concentrations and temperature ranges of gas release have been determined; the thermodynamic desorption parameters have been obtained; the relation between the form of the deuterium gas release curve and the phase state of the metal-deuterium system has been established.
Titanium films with 8 ~ 100-110 nm obtained by electronic beam deposition on KCl shearing distortions in the vacuum of P < 1x10"7 Pa have been used for structural investigations. After deposition, the films were released from the substrate in distilled water and caught on nickel disks 0 3 mm with an opening 0 1.2 mm in the centre. In such a form, the samples were the test objects to be studied using an electronic microscope. The samples were irradiated at an ambient temperature by D + ions with the energy of 20 keV up to doses ranging within 1 • 1016 to 5-1018 D/cm2. After irradiation, each sample was placed in an electronic microscope for the identification of phases obtained by implantation of D + ions, then the samples were subjected to annealing directly in the electronic microscope column, and the behaviour of their structure was traced at various heating temperatures. The investigation was carried out in two annealing modes: 1) linear rise of the film temperature from ~300 K to ~1000 K at the rate of ~2 K-s-1; 2) under conditions of isothermal annealing at the temperatures of ~630K and ~730 K.
For thermodesorption studies there were used the foils with the thickness of 8 = 0.3 mm.
The research results showed that deuterium implantation into titanium led to structural changes in the implanted layer. The structural transition nature is purely chemical. The structural transition is caused by the generation of titanium hydride TiD2. The high-temperature annealing of irradiated films leads to the decomposition of TiD2 hydride arising as a result of the ion implantation. The decomposition process begins at the
temperature of ~500 K and finishes at ~600 K with a sufficient duration of heating. In the course of TiD2 decomposition it is formed the intermediate crystalline structure, i. e. the P-phase of titanium with the BCC (body-centred cubic) lattice.
In case of short-term heating/cooling cycles within the temperature range of 300-580 K, the multiple repetition of the reversible transition HCP + FCC o o BCC + HCP (FCC = face-centred cubic) is observed, which is terminated by complete restoration of the FCC phase of a-Ti as a result of deuterium release from the sample.
The thermodesorption spectrum structure is a function of the implantation dose. If the irradiation doses range as 11016...71016 D/cm2, only one peak of gas release is observed with the maximum at T~ 1350 K. Further increase of the implantation dose leads to the increase of the amplitude of this peak and to the appearance of a new peak with Tm~ 800 K, as well as a weak-resolution peak having small amplitude with the maximum temperature of ~ 1100 K. With a dose exceeding ~ 2x1018 cm-2, a maximum peak appears in the spectrum at the temperature of ~ 200 K and grows further.
The comparison of electronographic data for crystalline structure of titanium with implanted deuterium ions with the thermosorption spectra bring at the conclusion that the nature of most gas release peaks is connected with phase transitions in a titanium-deuterium system. The peak with a maximum at Tm~ 800 K corresponds to the titanium hydride decomposition, and the temperature of the second peak maximum (~ 1100 K) almost coincides with the maximum temperature of the phase transition of a-titanium with the HCP lattice into P-titanium (BCC). The peak with T ~ 1350 K corresponds to the decomposition of a solid deuterium solution in titanium that is characterized by a full deuterium release into the gaseous phase. The peak with a maximum at ~ 200 K is connected with the deuterium component desorption exceeding its concentration in the TiD2 compound, i. e. this peak is connected with the formation of superstoichiometric deuterium from titanium hydride (decomposition of the solid solution of deuterium in titanium hydride TiD2).
